1. Field of the Invention
The present invention relates to a method for producing a semiconductor device by employing a laser annealing method. More particularly, the invention relates to a technique for improving crystallization of an amorphous semiconductor film or crystalline characteristics by a laser beam.
2. Description of the Related Art
A technique, which an amorphous semiconductor film formed on a substrate such as a glass is crystallized by a laser annealing method, has been developed. The laser annealing method described in the specification includes a technique re-crystallizing a damaged layer or an amorphous layer which is formed in a semiconductor substrate or a semiconductor film, a technique crystallizing an amorphous semiconductor film formed on a substrate, or a technique improving crystalline characteristics of a semiconductor film (crystalline semiconductor film) having a crystal structure. In a laser oscillation machine applied to laser annealing of a semiconductor, gas lasers, typically an excimer laser or solid state lasers, typically a YAG laser is used usually for the laser annealing.
An example of conventional laser annealing methods is disclosed in Japanese Patent Laid-Open No. 2-181419 which is a method irradiating a laser beam uniformly over an irradiated substance, a method scanning a spot shape of beam is disclosed in Japanese Patent Laid-Open No. 62-104117, and an irradiation method in which a laser treatment machine deforms a beam in line shape by an optical system is disclosed in Japanese Patent Laid-Open No. 8-195357.
In above Japanese Patent Laid-Open No. 62-104117, a technique is that scanning rate of a laser beam is set at not lower than a beam spot diameter×5000 /sec, and poly-crystallization of an amorphous semiconductor film is performed without the amorphous semiconductor film completely melted. A technique which a substantial single crystal region is obtained in such a way of irradiation of an extended laser beam to a semiconductor region formed in an island shape is disclosed in U.S. Pat. No. 4,330,363.
One of the features of the laser annealing method is that only a region absorbing energy of the laser beam can be selectively heated unlike an annealing method utilizing radiant heating or conductive heating. For example, a laser annealing using an excimer laser heats selectively and locally a semiconductor film to perform crystallization of a semiconductor film or activation treatment with little thermal damage to a glass substrate.
Active application of the laser annealing in recent years is focused on a formation of the poly-crystalline silicon film on a glass plate, the technique is applied to a formation of a thin film transistor (TFT) which is utilized for a switching element of a liquid crystal display apparatus. Use of the excimer laser effects thermal influence only to a region where the semiconductor film is formed so that a low cost glass substrate can be used.
TFT made from the crystallized poly-crystalline silicon film by the laser annealing can be driven at relatively high frequency, which enables the TFT not only to be provided in a pixel element as a switching element but also to be formed on a glass substrate as a driving circuit. A design rule of a pattern is in the order of 5 to 20 μm, the order of 106 to 107 of each TFT are formed in the driving circuit and pixel portion on the glass substrate respectively.
Crystallization of an amorphous silicon film by using the laser annealing method is achieved through a process of melting-solidification, and in particular it is considered that the crystallization consists of a crystalline nucleation stage and a stage of crystal growth from the crystalline nucleus. However, the crystallization by using a pulsed laser beam can not control a location of nucleation and nucleation density, which causes a spontaneous crystalline nucleus to be expected in the present circumstance. Consequently, a crystal grain is created at an optional location over the glass substrate, its size as small as the order of 0.2 to 0.5 μm can be only obtained. Grain boundary usually includes many crystal defects so that the crystal defects are considered as a factor of limitation of electric field effect mobility of TFT.
It is said that a non-melting region is formed in the pulsed laser annealing. In the pulsed laser annealing, larger grain size of the crystal can not be realized, because the crystal growth caused by the crystalline nucleus is dominant. In concrete, the crystal in which the grain boundary does not exist in a channel region of TFT and the crystal regarded substantially as a single crystal in a view of an element level can not be formed.
A created defect and dislocation in not only a grain boundary but also any other locations are caused by shrinkage of a film due to denseness in case of crystallization. Especially the defect in case of retraction of volume is pointed out that the defect is generated in an outer portion when a semiconductor film divided in an island shape is crystallized by a laser annealing method.
On the other hand, a method which the crystallization is achieved through a process of melting-solidification by scanning a continuous wave laser beam is considered to be close to zone melting method and to be able to realize a larger grain size by a continuous crystal growth. A problem is that quality of the finally obtained crystal depends on crystalline characteristics of a region which is crystallized at first to become a seed.
A wavelength of laser beams being able to heat a semiconductor film exists in a wide range from an ultraviolet region to an infrared region, and it is considered that the laser beam having the wavelength in the range from an ultraviolet region to a visible light region is applicable from a viewpoint of absorption coefficient of the semiconductor when a semiconductor film formed on a substrate or a semiconductor region separately formed is heated selectively. However, light of a solid state laser which can obtain relatively high power even in a visible light region generates interference on a irradiated surface because of long coherent length, which causes uniform irradiation of the laser beam to be difficult.
Crystallization by the continuous wave laser beam having longer time melting state than the pulsed laser beam increases a ratio being taken impurity in a crystal from the outside, and its segregation causes the defect to be generated, even though the crystalline characteristics are improved. Consequently, a problem is that quality of crystal becomes worse.
In view of the foregoing, it is an object of the invention to provide a technique forming a crystalline semiconductor film whose orientation is uniform by control of crystalline orientation and obtaining a crystalline semiconductor film in which a concentration of an impurity is reduced.